Shear-induced dilation of confined liquid films
Shear-induced dilation of confined liquid films
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受限液膜的剪切引起的膨胀
DOI：
10.1023/a:1018852326571
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发表时间：
2000
影响因子：
3.2
通讯作者：
S. Granick
中科院分区：
文献类型：
--
作者：
A. Dhinojwala;S. Bae;S. Granick
我们证明，剪切的分子薄流体膜在粘滑点（即从静摩擦到动摩擦的转变）处膨胀，表明发生滑动时密度降低。这与散装流体变形时的不可压缩特性形成对比。膨胀的幅度小于分子的大小，并且在聚合物系统（大分子）中比小分子流体更大。实验采用了经过修改的表面力装置，以使用压电方法测量动态剪切激励期间薄膜厚度的亚埃变化，动态剪切激励的执行速度太快，以致于在剪切激励期间不允许流体进入和离开剪切接触区域。为了证明膨胀效应的普遍性，研究的具体系统包括复杂程度各异的非极性流体（球状分子，OMCTS；支链烷烃，角鲨烷；系链二嵌段共聚物，聚乙烯吡啶-聚丁二烯）以及水性电解质，溶解在水中的氯化镁。还对压电方法进行了广泛的分析，该方法用于检测太小而无法通过多光束干涉测量方法观察到的体积变化，而多光束干涉测量方法是表面力设备中传统的厚度测量方法。
We demonstrate that sheared molecularly-thin fluid films dilate at the point of stick-to-slip (which is the transition from static to kinetic friction), indicating that density decreases when sliding occurs. This contrasts with incompressibility characteristic of bulk fluids when they are deformed. The magnitude of dilation was less than the size of the molecule and was larger in a polymer system (large molecules) than for small-molecule fluids. The experiments employed a surface forces apparatus modified to measure, using piezoelectric methods, sub-angstrom variations of film thickness during dynamic shear excitations that were performed at rates too rapid to allow fluid to enter and exit the zone of shear contact during the period of shear excitation. To demonstrate generality of the dilation effect, the specific systems studied included nonpolar fluids whose complexity was varied (a globular-shaped molecule, OMCTS; a branched alkane, squalane; a tethered diblock copolymer, polyvinylpyridine–polybutadiene) and also an aqueous electrolyte, MgCl2 dissolved in water. Extensive analysis is also presented of the piezoelectric methods that were employed to detect volume changes too small to observe by the methods of multiple beam interferometry that are traditional for thickness measurement in a surface forces apparatus.